Conventional position-measuring devices for measuring the position of two objects that are movable relative to each other along at least one measuring direction are made up of a measuring standard which is connected to one of the two objects, as well as a scanning unit which is connected to the other one of the two objects. The measuring standard includes an incremental graduation which extends in the measuring direction. The scanning unit has a light source, one or more grating(s), as well as a detector system. A plurality of groups of detector elements, which are disposed in a detection plane, is provided on the side of the detector system. If the two objects execute a relative movement, a plurality of position-dependent, phase-shifted scanning signals can be generated by scanning a periodic fringe pattern that results in the detection plane. The detector elements that have in-phase scanning signals form a group in each case. Four preferably rectangular detector elements are typically provided in the detector system for so-called single-field scanning for the purpose of generating four scanning signals that have an individual phase offset of 90° within one period of the fringe pattern. Such single-field scanning is considered to be advantageous even in instances where the operating conditions are less than ideal, such as when the scanned measuring standard is contaminated, inasmuch as only a slight detrimental effect is noticeable on the ideal signal shape of the scanning signals.
As the resolution of such position-measuring devices increases, the periods of the resulting fringe pattern in the detection plane become smaller and smaller. However, there are technological limits in regard to the detector elements as far as their minimally possible width is concerned. At a provided spacing of adjacent detector elements of 5 μm and a minimally possible width of a detector element of 5 μm, such conventional single field scanning is therefore no longer usable for scanning fringe pattern periods below 40 μm. Moreover, the portion of the margin capacities in relation to the total capacity of the individual detector elements increases as the detector element structures become smaller and smaller. Indicating suitable measures also for high-resolution optical position-measuring devices so as to ensure excellent signal quality even under less than optimal operating conditions constitutes a problem.
To solve this problem, it is conventional to place diaphragm structures in front of the detector system, which include periodically placed, transparent regions, whose periodicity is less than the dimensions of an individual detector element in the measuring direction. In this context, reference is made, for example, to FIGS. 2 and 3 in the publication of J. Carr et al. bearing the title “Miniaturized Optical Encoder for Ultra Precision Metrology Systems” in Precision Engineering 33 (2009), p. 263-267. However, this approach has the problem of requiring the diaphragm structures to be aligned extremely precisely in relation to the detector elements if the detector elements are small. Otherwise low quality scanning signals will result, i.e., signals having a lower modulation degree and thus lower signal amplitude. If larger detector elements are involved, on the other hand, the adjustment of the diaphragm structures is less critical. Nevertheless, under less than ideal operating conditions, good signal quality can no longer be guaranteed.